This invention relates to an exhaust gas recirculation control system for internal combustion engines, which is capable of effecting exhaust gas recirculation control in a prompt and accurate manner through feedback control of the valve opening of an exhaust gas recirculation valve.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by U.S. Ser. No. 8,205,940 assigned to the same assignee as the present application, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the above valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to the above proposed fuel supply control system, the basic value of the valve opening period of the fuel injection device is determined by the use of two different engine rpm-intake pipe absolute pressure maps which are selectively used depending upon whether the exhaust gas recirculation valve is operating or at rest. On the other hand, the exhaust gas recirculation valve has its valve lifting amount determined as a function of engine rpm, intake pipe absolute pressure, etc. for control of the exhaust gas recirculation quantity. In these manners, the exhaust gas recirculation quantity and the fuel supply quantity are controlled in dependence on operating conditions of the engine, so as to achieve optimum emission characteristics and driveability of the engine.
On the other hand, an exhaust gas recirculation control system has been proposed e.g. by U.S. Pat. No. 4,164,032, in which valve opening or lift command values for the exhaust gas recirculation valve are previously determined as required values, which are functions of operating conditions of the engine, the actual valve opening or lift value is detected, and the valve opening or lift value of the exhaust gas recirculation valve is controlled in a feedback manner so as to minimize the difference between a valve opening or lift command value and an actual detected value. In such proposed system, the exhaust gas recirculation valve is generally adapted to be closed or opened by an actuator which is responsive to the difference between negative pressure in the intake pipe and atmospheric pressure. Therefore, if the valve opening correcting speed of the actuator is not appropriate, the valve opening of the exhaust gas recirculation valve cannot be exactly controlled to a required value. For instance, if the correcting speed is too high, the actuator and the exhaust gas recirculation valve coupled thereto undergo overshooting or hunting, while if the correcting speed is too low, sufficiently high control responsiveness cannot be obtained. Thus, accurate control of the exhaust gas recirculation quantity is not feasible with inappropriate valve opening correcting speeds.